1. Field of the Invention
The invention relates in general to an organic light emitting display, and more particularly to an organic light emitting diode circuit for such a display.
2. Description of the Related Art
An organic light emitting diode circuit used in organic light emitting display normally stores signals for controlling the luminance of an organic light emitting diode (OLED) via thin film transistors (TFTs) and capacitors. However, the above TFTs, after prolonged use, will exhibit a threshold voltage (Vth) shift. This shift amount is related to the operation time of the TFTs and the current flowing therethrough.
In a display process, owing that the TFT for driving an OLED for each diode has a different current when turned on, and the driving TFTs will have different shift amounts of threshold voltage. As a result, the luminance of each diode will not have the same correspondence relation as the received pixel data, which in turn results in an uneven frame display.
In order to solve the above issue, a voltage compensation technique is applied to a conventional organic light emitting diode. Referring to FIG. 1, a circuit diagram of a conventional organic light emitting diode is shown. An organic light emitting diode circuit 100 includes TFTs MP1˜MP5, a storage capacitor Cst and an organic light emitting diode (OLED). The TFT MP3 is controlled by a scan signal Scan and the TFT MP1 is coupled between the TFT MP3 and the storage capacitor Cst. The TFT MP2 drives the OLED to illuminate according to a voltage of the storage capacitor Cst when the TFT MP4 is turned on. The TFT MP5 is controlled by a reset signal Rst and the TFT MP4 is controlled by an enable signal Enb.
The conventional organic light emitting diode circuit 100, as written pixel data Vdata, uses the TFT MP1 with the same device feature as the TFT MP2 to cancel the threshold voltage Vth2 of the TFT MP2. More specifically, when the scan signal Scan is enabled, the TFT MP3 is turned on and the pixel data Vdata charges the storage capacitor Cst via the TFTs MP3 and MP1. In the meanwhile, due to a voltage compensation feature of the TFT MP1, the voltage level at point X, that is, a gate voltage of the TFT MP2, is lower than a voltage level at point Y, that is, a threshold voltage Vth1 of the TFT MP1, and thus the voltage difference between the source and gate of the TFT MP2 is increased by Vth1. The threshold voltage Vth1 is substantially the same as the threshold voltage Vth2, and the voltage difference between the source and gate of the TFT MP2 is just equal to difference between Vdd and the pixel voltage Vdata. Therefore, the current IOLED flowing by the OLED is precisely related to the pixel voltage Vdata.
In the above compensation technique, the compensation operation is performed during a data writing stage to eliminate errors generated from the threshold voltage Vth2. However, recent OLED panels tend to be developed in high resolution and large size. As a result, the time for writing data is greatly reduced. However, the TFT MP1 has small current as it is turned on, and thus it needs longer compensation time, which will result in an irregular operation of the TFT MP1 and disability of the compensation mechanism. However, with the circuit design of a conventional voltage compensation arrangement, it is essential that the nodes X and Y both have a stable voltage state during the “data writing” stage, otherwise, a charge sharing issue will be generated at the frame display stage. Accordingly, the conventional voltage compensation arrangement is apt to exhibit the drawback that the the node X does not reach a stable voltage state for canceling Vth2 due to inadequate time. In this situation, the TFT MP1 is still turned on. As a result, the charge sharing issue is generated and the display luminance can not reach the predicted luminance in correspondence with the pixel voltage Vdata.